The four-minute warning was a signal for a nuclear attack.
four-minute warning
idiomA UK government public alert system in use during the Cold War to advise of an imminent nuclear attack.
During the Cold War, citizens lived in fear that the four-minute warning would suddenly sound across the country.
The government developed the four-minute warning as a desperate measure to alert the public of an incoming missile strike, though many doubted it offered enough time to seek safety.